This document discusses error management in ZIO compared to Future. It begins with an overview of ZIO and Future effects before comparing how each handles errors. Key differences noted are that Future throws errors away on a side channel while ZIO composes errors. The document recommends best practices for error handling in ZIO like extending exceptions in sealed traits and avoiding reflexive logging. It concludes by discussing how ZIO enables next-generation debugging by tracking fibers and continuations during asynchronous execution.

1. 1
Error Management
ZIO vs Future
Dublin Scala Meetup, May 11th
John A. De Goes @jdegoes
Kai @kaidaxofficial
...with help of Pavel Shirshov @shirshovp

2. Agenda
A Tale of Two Effects
2
Next-Gen Debugging Conclusion
Managing Errors

3. 3
A TALE OF
TWO EFFECTS
FUTURE

4. Parallel
Future enables parallel
computations and
non-blocking gathering.
Error
Future has a built-in error
channel for
Throwable-based errors.
Eager
Future is not referentially
transparent; refactoring
may change behavior.
4
FUTURE
Async
Future enables
non-blocking code that
efficiently uses threads.

5. 5
Future ZIO Effect
go running

6. Procedural Functional
def program: Unit = {
println("What’s your name?")
val name = readLine()
println(s"Howdy. $name!")
return ()
}
val program = for {
_ <- putStrLn("What’s your name?")
name <- getStrLn
_ <- putStrLn(s"Howdy, $name")
} yield ()

7. 7
Future ZIO
Performance > 100x slower > 100x faster
Cancellation & Timeouts 𐄂 ✔
Effect Combinators 𐄂 ✔
Resource Safety 𐄂 ✔
Fiber Concurrency 𐄂 ✔
Equational & Type Reasoning 𐄂 ✔
Testability 𐄂 ✔
Error Management & Debugging ? ?

8. 8
ZIO[R, E, A]
Environment Type
Failure Type
Success Type

9. 9
ZIO[R, E, A]
~
R => Either[E, A]

10. 10
type Task [ +A] = ZIO[Any, Throwable, A]
type UIO [ +A] = ZIO[Any, Nothing, A]
type TaskR[+R,+A] = ZIO[ R, Throwable, A]
type IO [+E,+A] = ZIO[Any, E, A]

11. 11
type Task [ +A] = ZIO[Any, Throwable, A]
type UIO [ +A] = ZIO[Any, Nothing, A]
type TaskR[+R,+A] = ZIO[ R, Throwable, A]
type IO [+E,+A] = ZIO[Any, E, A]

12. 12
type Task [ +A] = ZIO[Any, Throwable, A]
type UIO [ +A] = ZIO[Any, Nothing, A]
type TaskR[+R,+A] = ZIO[ R, Throwable, A]
type IO [+E,+A] = ZIO[Any, E, A]

13. 13
type Task [ +A] = ZIO[Any, Throwable, A]
type UIO [ +A] = ZIO[Any, Nothing, A]
type TaskR[+R,+A] = ZIO[ R, Throwable, A]
type IO [+E,+A] = ZIO[Any, E, A]

14. ZIO[R, E, A]
Synchronous Asynchronous Errors Resource
ZIO.succeed(…)
ZIO.effect(…)
ZIO.effectTotal(…)
effectBlocking(…)
ZIO.effectAsync(…)
ZIO.effectAsyncMaybe(…)
ZIO.effectAsyncInte…(…)
ZIO.fromFuture(…)
ZIO.fail(…)
ZIO.fromOption(…)
ZIO.fromEither(…)
ZIO.fromTry(…)
ZIO.bracket(…)
ZIO.reserve(…)
ZIO.ensuring(…)
fromAutoCloseable(…)

15. 15
ERROR
MANAGEMENT
FUTURE

16. Fail Domain errors, business errors,
transient errors, expected errors...
Expected Errors
Not Reflected in Types
DieSystem errors, fatal errors,
unanticipated errors, defects...
Unexpected Errors
Reflected in Types
16
ERROR DUALITY

17. 17
ERROR DUALITY
Not Reflected in TypesNot Reflected in Types
val failed: Future[Nothing] =
Future.failed(new Exception)
val died: Future[Nothing] =
Future(throw new Error)
FUTURE

18. 18
ERROR DUALITY
Not Reflected in TypesReflected in Types
val failed: IO[String, Nothing] =
ZIO.fail(“Uh oh!”)
val died: IO[Nothing, Nothing] =
ZIO.dieMessage(“Uh oh!”)

19. 19
ERROR COMPOSITION
e1
e2t
First Error

20. 20
ERROR COMPOSITION
e1
e2t
Second Error

21. 21
ERROR COMPOSITION
e1
e2tFinalizer Error

22. 22
ERROR COMPOSITION
e1
e2t
?

23. 23
ERROR COMPOSITION
e1
e2t
FUTURE
Thrown away!!!Reported on side channel!!!

24. 24
ERROR COMPOSITION
e1
e2t
Cause[E]

25. 25
ERROR COMPOSITION
e1
e2t
Cause[E]
Cause.Fail(e1)

26. 26
ERROR COMPOSITION
e1
e2t
Cause[E]
Cause.Both(
Cause.Fail(e1),
Cause.Fail(e2))

27. 27
ERROR COMPOSITION
e1
e2t
Cause[E]
Cause.Both(
Cause.Fail(e1),
Cause.Then(
Cause.Fail(e2),
Cause.Die(t))

28. zio.FiberFailure: Fiber failed.
╥
╠══╗
║ ║
║ ║
║ ║
║ ╠─ A checked error was not handled:
║ ║
Failed(DatabaseUnreachableError)
║ ║
║ ╠─ A finalizer threw an error:
║ ▼
Die(IndexOutOfBoundsException())
║
╠─ A checked error was not handled:
▼ Failed(UserIdNotFoundError) 28
ERROR COMPOSITION
e2
t
e1

29. Asynchronous
29
ERROR PROPAGATION
Synchronous
ConcurrentParallel
Resource

30. Asynchronous
30
ERROR PROPAGATION
Synchronous
ConcurrentParallel
Resource
FUTURE

31. Asynchronous
31
ERROR PROPAGATION
Synchronous
ConcurrentParallel
Resource

32. 32
ERROR RECOVERY
Fallback Catching Folding Value
ZIO#orElse(…)
ZIO#orElseEither(…)
ZIO#catchAll(…)
ZIO#catchSome(…)
ZIO#fold(…, …)
ZIO#foldM(…, …)
effect1.orElse(effect2) future1.fallback(future2)
ZIO#either
ZIO#run
FUTURE

33. 33
ERROR RECOVERY
Fallback Catching Folding Value
ZIO#orElse(…)
ZIO#orElseEither(…)
ZIO#catchAll(…)
ZIO#catchSome(…)
ZIO#fold(…, …)
ZIO#foldM(…, …)
effect.catchAll(f) future.recoverWith(pf)
ZIO#either
ZIO#run
FUTURE

34. 34
ERROR RECOVERY
Fallback Catching Folding Value
ZIO#orElse(…)
ZIO#orElseEither(…)
ZIO#catchAll(…)
ZIO#catchSome(…)
ZIO#fold(…, …)
ZIO#foldM(…, …)
effect.catchSome(pf) future.recoverWith(pf)
ZIO#either
ZIO#run
FUTURE

35. 35
ERROR RECOVERY
Fallback Catching Folding Value
ZIO#orElse(…)
ZIO#orElseEither(…)
ZIO#catchAll(…)
ZIO#catchSome(…)
ZIO#fold(…, …)
ZIO#foldM(…, …)
effect.fold(err, succ)
ZIO#either
ZIO#run
FUTURE

36. 36
ERROR RECOVERY
Fallback Catching Folding Value
ZIO#orElse(…)
ZIO#orElseEither(…)
ZIO#catchAll(…)
ZIO#catchSome(…)
ZIO#fold(…, …)
ZIO#foldM(…, …)
effect.foldM(err, succ)
ZIO#either
ZIO#run
FUTURE

37. 37
ERROR RECOVERY
Fallback Catching Folding Value
ZIO#orElse(…)
ZIO#orElseEither(…)
ZIO#catchAll(…)
ZIO#catchSome(…)
ZIO#fold(…, …)
ZIO#foldM(…, …)
effect.either
ZIO#either
ZIO#run
FUTURE

38. 38
ERROR RECOVERY
Fallback Catching Folding Value
ZIO#orElse(…)
ZIO#orElseEither(…)
ZIO#catchAll(…)
ZIO#catchSome(…)
ZIO#fold(…, …)
ZIO#foldM(…, …)
ZIO#either
ZIO#run
effect.run
FUTURE

39. 39
BEST
PRACTICES

40. 40
1. DON’T TYPE UNEXPECTED ERRORS
ZIO.effect(httpClient.get(url)).refineOrDie {
case e : TemporarilyUnavailable => e
}.retry(RetryPolicy).orDie
: IO[TemporarilyUnavailable, Response]

41. 41
2. DO EXTEND EXCEPTION WITH SEALED TRAITS
sealed trait UserServiceError
extends Exception
case class InvalidUserId(id: ID)
extends UserServiceError
case class ExpiredAuth(id: ID)
extends UserServiceError
UserServiceError
InvalidUserId ExpiredAuth

42. 42
2. DO EXTEND EXCEPTION WITH SEALED TRAITS
userServiceError match {
case InvalidUserId(id) => ...
case ExpiredAuth(id) => ...
}

43. 43
2. DO EXTEND EXCEPTION WITH SEALED TRAITS
for {
service <- userAuth(token)
_ <- service.userProfile(userId)
body <- generateEmail(orderDetails)
receipt <- sendEmail(“Your Order Details”,
body, profile.email)
} yield receipt
ExpiredAuth

44. 44
2. DO EXTEND EXCEPTION WITH SEALED TRAITS
for {
service <- userAuth(token)
_ <- service.userProfile(userId)
body <- generateEmail(orderDetails)
receipt <- sendEmail(“Your Order Details”,
body, profile.email)
} yield receipt
InvalidUserId

45. 45
2. DO EXTEND EXCEPTION WITH SEALED TRAITS
for {
service <- userAuth(token)
_ <- service.userProfile(userId)
body <- generateEmail(orderDetails)
receipt <- sendEmail(“Your Order Details”,
body, profile.email)
} yield receipt
Nothing

46. 46
2. DO EXTEND EXCEPTION WITH SEALED TRAITS
for {
service <- userAuth(token)
_ <- service.userProfile(userId)
body <- generateEmail(orderDetails)
receipt <- sendEmail(“Your Order Details”,
body, profile.email)
} yield receipt
EmailDeliveryError

47. 47
2. DO EXTEND EXCEPTION WITH SEALED TRAITS
for {
service <- userAuth(token)
_ <- service.userProfile(userId)
body <- generateEmail(orderDetails)
receipt <- sendEmail(“Your Order Details”,
body, profile.email)
} yield receipt
IO[Exception, Receipt]

48. 48
3. DON’T REFLEXIVELY LOG ERRORS
uploadFile(“contacts.csv”).catchAll { error =>
// Log error and re-fail:
for {
_ <- logger.error(error)
_ <- ZIO.fail(error)
} yield ()
}

49. 49
4. DO GET TO KNOW UIO
type UIO[+A] = ZIO[Any, Nothing, A]
Cannot fail!

50. lazy val processed: UIO[Unit] =
processUpload(upload).either.flatMap {
case Left (_) => processed.delay(1.minute)
case Right(_) => ZIO.succeed(())
}
50
4. DO GET TO KNOW UIO
Fails with UploadError

51. lazy val processed: UIO[Unit] =
processUpload(upload).either.flatMap {
case Left (_) => processed.delay(1.minute)
case Right(_) => ZIO.succeed(())
}
51
4. DO GET TO KNOW UIO
Fails with Nothing

52. 52
4. DO GET TO KNOW UIO
lazy val processed: UIO[Unit] =
processUpload(upload).either.flatMap {
case Left (_) => processed.delay(1.minute)
case Right(_) => ZIO.succeed(())
}
Fails with Nothing

53. NEXT-GENERATION
DEBUGGING

54. 54
def asyncDbCall(sql: SQL): Future[Result]
def selectHumans(): Future[Result] = ...asyncDbCall(...)...
def selectPets(): Future[Result] = ...asyncDbCall(...)...
FUTURE

55. 55
def asyncDbCall(sql: SQL): Future[Result]
def selectHumans(): Future[Result] = ...asyncDbCall(...)...
def selectPets(): Future[Result] = ...asyncDbCall(...)...
FUTURE
Which function failed, selectHumans or selectPets?
PostgresException: Syntax error at or near 42
at example$.getConnection(example.scala:43)
at example$.$anonfun$asyncDbCall$1(example.scala:23)
at scala.concurrent.Future$.$anonfun$apply$1(Future.scala:658)
at scala.util.Success.$anonfun$map$1(Try.scala:255)
at scala.util.Success.map(Try.scala:213)
at scala.concurrent.Future.$anonfun$map$1(Future.scala:292)
at scala.concurrent.impl.Promise.liftedTree1$1(Promise.scala:33)
at scala.concurrent.impl.Promise.$anonfun$transform$1(Promise.scala:33)
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:64)
at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

56. 56
Which function failed, selectHumans or selectPets?
FUTURE
def asyncDbCall(sql: SQL): Future[Result]
def selectHumans(): Future[Result] = ...asyncDbCall(...)...
def selectPets(): Future[Result] = ...asyncDbCall(...)...
Only the last operation is mentioned
PostgresException: Syntax error at or near 42
at example$.getConnection(example.scala:43)
at example$.$anonfun$asyncDbCall$1(example.scala:23)
at scala.concurrent.Future$.$anonfun$apply$1(Future.scala:658)
at scala.util.Success.$anonfun$map$1(Try.scala:255)
at scala.util.Success.map(Try.scala:213)
at scala.concurrent.Future.$anonfun$map$1(Future.scala:292)
at scala.concurrent.impl.Promise.liftedTree1$1(Promise.scala:33)
at scala.concurrent.impl.Promise.$anonfun$transform$1(Promise.scala:33)
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:64)
at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

57. 57
Which function failed, selectHumans or selectPets?
FUTURE
Only the last operation is mentioned
There is NO way to know!!!
def asyncDbCall(sql: SQL): Future[Result]
def selectHumans(): Future[Result] = ...asyncDbCall(...)...
def selectPets(): Future[Result] = ...asyncDbCall(...)...
PostgresException: Syntax error at or near 42
at example$.getConnection(example.scala:43)
at example$.$anonfun$asyncDbCall$1(example.scala:23)
at scala.concurrent.Future$.$anonfun$apply$1(Future.scala:658)
at scala.util.Success.$anonfun$map$1(Try.scala:255)
at scala.util.Success.map(Try.scala:213)
at scala.concurrent.Future.$anonfun$map$1(Future.scala:292)
at scala.concurrent.impl.Promise.liftedTree1$1(Promise.scala:33)
at scala.concurrent.impl.Promise.$anonfun$transform$1(Promise.scala:33)
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:64)
at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

58. 58
Asynchronous
def myQuery =
for {
_ <- UIO(println(“Querying!”))
res <- queryDatabase
} yield res

59. 59
Failure!
def myQuery =
for {
_ <- UIO(println(“Querying!”))
res <- queryDatabase
} yield res

60. 60
def myQuery =
for {
_ <- UIO(println(“Querying!”))
res <- queryDatabase
} yield res
Fiber:0 ZIO Execution trace:
at myQuery(example.scala:4)
at myQuery(example.scala:3)
Fiber:0 was supposed to continue to:
a future continuation at myQuery(example.scala:5)
Failure!

61. 61
def myQuery =
UIO(println(“Querying!”))
.flatMap(_ =>
queryDatabase
.map(res => res))
Fiber:0 ZIO Execution trace:
at myQuery(example.scala:4)
at myQuery(example.scala:3)
Fiber:0 was supposed to continue to:
a future continuation at myQuery(example.scala:5)

62. 62
def myQuery =
UIO(println(“Querying!”))
.flatMap(_ =>
queryDatabase
.map(res => res))
Fiber:0 ZIO Execution trace:
at myQuery(example.scala:4)
at myQuery(example.scala:3)
Fiber:0 was supposed to continue to:
a future continuation at myQuery(example.scala:5)
The Past

63. 63
def myQuery =
UIO(println(“Querying!”))
.flatMap(_ =>
queryDatabase
.map(res => res))
Fiber:0 ZIO Execution trace:
at myQuery(example.scala:4)
at myQuery(example.scala:3)
Fiber:0 was supposed to continue to:
a future continuation at myQuery(example.scala:5)
The Past
The Future

64. 64
def asyncDbCall(sql: SQL): Task[Result]
val selectHumans: Task[Result] = ...asyncDbCall(...)...
val selectPets: Task[Result] = ...asyncDbCall(...)...

65. 65
def asyncDbCall(sql: SQL): Task[Result]
val selectHumans: Task[Result] = ...asyncDbCall(...)...
val selectPets: Task[Result] = ...asyncDbCall(...)...
Fiber:0 ZIO Execution trace:
at asyncDbCall(example.scala:22)
at selectHumans(example.scala:26)
Fiber:0 was supposed to continue to:
a future continuation at selectHumans(example.scala:27)

66. 66
def asyncDbCall(sql: SQL): Task[Result]
val selectHumans: Task[Result] = ...asyncDbCall(...)...
val selectPets: Task[Result] = ...asyncDbCall(...)...
Fiber:0 ZIO Execution trace:
at asyncDbCall(example.scala:22)
at selectHumans(example.scala:26)
Fiber:0 was supposed to continue to:
a future continuation at selectHumans(example.scala:27)
Gotcha!

67. 67
EXECUTION TRACES
def doWork(condition: Boolean) = {
if (condition) {
doSideWork()
}
doMainWork()
}
java.lang.Exception: Worker failed!
at example$.doMainWork(example.scala:54)
at example$.doWork(example.scala:50)
at example$$anon$1.run(example.scala:60)
No mention of doSideWork()
PROCEDURAL

68. 68
EXECUTION TRACES
def doWork(condition: Boolean) =
for {
_ <- IO.when(condition)(doSideWork)
_ <- doMainWork
} yield ()
Fiber:0 ZIO Execution trace:
at example$.doMainWork(example.scala:27)
at example$.doWork(example.scala:23)
at example$.doSideWork(example.scala:26)
The conditional was true!

69. 69
CONCURRENT TRACES
uploadUsers uploadPets
uploadTo(target)
error!
def uploadUsers(users: List[User]): Task[Unit] =
IO.foreachPar_(users.map(toJSON))(uploadTo(dest1))
def uploadPets(pets: List[Pet]): Task[Unit] =
IO.foreachPar_(pets.map(toJSON))(uploadTo(dest2))
def uploadTo(dest: URL)(json: JSON): Task[Unit] = ...

70. 70
CONCURRENT TRACES
uploadUsers uploadPets
uploadTo(target)
error!
java.lang.Exception: Expired credentials
at example$.$anonfun$uploadTo$1(example.scala:28)
Fiber:1 ZIO Execution trace:
at example$.uploadTo(example.scala:28)
Fiber:1 was supposed to continue to: <empty trace>
Fiber:1 was spawned by:
╥
╠─ Fiber:0 ZIO Execution trace: <empty trace>
║
║ Fiber:0 was supposed to continue to:
║ example$.uploadUsers(example.scala:21)

71. 71
TAGLESS FINAL TRACES
Gain insights into FP libraries

72. 72
● Tracing is fast , impact is negligible for real apps
● > 50x faster than Future
● ...even on synthetic benchmarks!
● Impact can be limited by config
● Much lower than monad transformers [10x]
● Enabled by default, no Java agents, no ceremony
Disable if tracing is a hot spot
effect.untraced
MADE FOR
PRODUCTION

73. I
73
CONCLUSION
https://github.com/zio
https://gitter.im/zio/core
https://zio.dev
@jdegoes
@kaidaxofficial
@shirshovp